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Low-temperature Liquid Exfoliation of Milligram-scale Single Crystalline Few-layer β12-Borophene Sheets as Efficient Electrocatalysts for Lithium–Sulfur Batteries

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Abstract

Two-dimensional (2D) borophene is predicted as an ideal electrode material for lithium sulfur (Li-S) batteries because of low-density, metallic conductivity, high Li-ion surface mobility and strong interface bonding energy to polysulfide. But until now, 2D borophene-based Li-S batteries have not yet been achieved due to the absence of massive synthesis method. Herein, we developed a novel low-temperature liquid exfoliation (LTLE) method for scalable synthesis of single crystalline 2D few-layer β 12 -borophene sheets with a \(P\stackrel{-}{6}m2\) symmetry. The as-synthesized 2D sheets were used as the polysulfide immobilizers and electrocatalysts of Li-S batteries for the first time. The resulting Li-S cells employing borophene sheets delivered a strikingly high areal capacity of 5.2 mAh cm − 2 at a high sulfur loading of 7.8 mg cm − 2 with an ultralow capacity fading rate (0.039 % per cycle) in 1000 cycles, outperforming most of the Li-S batteries employing other 2D materials. Under the help of few-layer β 12 -borophene, their high-activity behaviors should be attributed to the significant enhancement of both the Li-ion’s surface migration and the adsorption energy for Li 2 S n clusters based on density functional theory (DFT) models. Our research reveals great potential of 2D β 12 -borophene sheets in future high-performance Li-S batteries.
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Low-temperature Liquid Exfoliation of Milligram-
scale Single Crystalline Few-layer β12-Borophene
Sheets as Ecient Electrocatalysts for Lithium–
Sulfur Batteries
Haojian Lin
Sun Yat-sen University
Haodong Shi
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Zhen Wang
Dalian Institute of Chemical Physics
Yue-Wen Mu
Shanxi University https://orcid.org/0000-0002-0162-5091
Si-Dian Li
Shanxi University https://orcid.org/0000-0001-5666-0591
Jijun Zhao
Dalian University of Technology
Jingwei Guo
Dalian Institute of Chemical Physics
Bing Yang ( byang@dicp.ac.cn )
Dalian Institute of Chemical Physics https://orcid.org/0000-0003-3515-0642
Zhong-Shuai Wu
Dalian Institute of Chemical Physics, Chinese Academy of Sciences https://orcid.org/0000-0003-1851-
4803
Fei Liu
Sun Yat-sen University https://orcid.org/0000-0001-7603-9436
Article
Keywords: Two-dimensional (2D) borophene, lithium sulfur (Li-S) batteries, low-temperature liquid
exfoliation (LTLE) method
DOI: https://doi.org/10.21203/rs.3.rs-209092/v1
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License: This work is licensed under a Creative Commons Attribution 4.0 International License. 
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Abstract
Two-dimensional (2D) borophene is predicted as an ideal electrode material for lithium sulfur (Li-S)
batteries because of low-density, metallic conductivity, high Li-ion surface mobility and strong interface
bonding energy to polysulde. But until now, 2D borophene-based Li-S batteries have not yet been
achieved due to the absence of massive synthesis method. Herein, we developed a novel low-temperature
liquid exfoliation (LTLE) method for scalable synthesis of single crystalline 2D few-layer
β
12-borophene
sheets with a symmetry. The as-synthesized 2D sheets were used as the polysulde
immobilizers and electrocatalysts of Li-S batteries for the rst time. The resulting Li-S cells employing
borophene sheets delivered a strikingly high areal capacity of 5.2 mAh cm− 2 at a high sulfur loading of
7.8 mg cm− 2 with an ultralow capacity fading rate (0.039 % per cycle) in 1000 cycles, outperforming
most of the Li-S batteries employing other 2D materials. Under the help of few-layer
β
12-borophene, their
high-activity behaviors should be attributed to the signicant enhancement of both the Li-ion’s surface
migration and the adsorption energy for Li2Sn clusters based on density functional theory (DFT) models.
Our research reveals great potential of 2D
β
12-borophene sheets in future high-performance Li-S batteries.
Introduction
The rapid development of electrochemical energy storage devices in the elds of electric vehicles,
portable electronic devices and large-scale smart power grids continuously drive the researchers to
explore lower cost, higher energy density, and better safety batteries than current lithium-ion batteries1-4.
Among many candidates, lithium sulfur (Li-S) batteries have been gaining the global attention due to their
overwhelming energy density (2600 Wh kg-1), natural abundance and environment-friendly of sulfur
feedstock5-9. However, the existence of internal polysulde shuttling, large volume expansion of sulfur
and sluggish redox kinetics inevitably lead to the sharp deterioration of the electrochemical performances
of Li-S batteries10-12.
Considering the irreversible loss and inecient utilization of sulfur cathodes, much effort has been
devoted to the design of advanced materials for immobilizing and activating sulfur materials, such as
transitional-metal oxides13,14, suldes15-17, carbides,18-20 metal nitrides21-23and heterostructures24-26.
Recently, two-dimensional (2D) materials with strong in-plane covalent bonds and weak interlayered van
der Waals (vdW) forces have been intensively studied because of their superior advantages over
traditional bulk materials for Li-S cell applications27,28. 2D materials such as siloxane29, black
phosphorene30,31, BN32,33, C3N434,35, and MXene36,37, were found to exhibit excellent catalytic activities
towards polysuldes because of their extraordinary surface properties. However, most of these 2D
material-based Li-S cells still have some disadvantages, such as low capacity38, slow charge-discharge
rate39,40, structure instability38,41, and poor cyclic stability32. Hence, the design and development of novel
2D materials are highly demanded towards high-performance Li-S batteries with large catalytic activity,
high-ecient adsorption, fast conversion of polysuldes and long-term durability.
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As a typical 2D Dirac material consisted of the lightest solid element, 2D borophene with unique surface
conguration and complex multicenter-two electron bonds has been earlier predicted as an ideal
electrode material for Li-S batteries due to its native metallic conductivity42, large elastic modulus43,
heavy anisotropy44, high Fermi velocity(6.6×105 m/s) 45, excellent thermal and chemical stability46, large
Li-ion surface mobility as well as strong bonding energy to polysulde clusters47. However, borophene-
based Li-S cells have not yet been achieved for practical use so far owing to the absence of a facile route
for the scalable production of 2D borophene nanomaterials.
In this work, we developed a low-temperature liquid exfoliation (LTLE) strategy for scalable production of
single crystalline borophene sheets as ecient polysulde electrocatalyst for Li-S batteries. Few-layer 2D
borophene sheets with
β
12-phase were thus identied with an average ake size of ~3 μm and an
ultrathin thickness less than 10 atomic layers. The
β
12-borophene sheets exhibited extraordinary
performances as ecient immobilizer and electrocatalyst for advanced Li-S batteries, showing excellent
rate performance of 721 mAh g-1at 8 C (1 C = 1675 mAh g-1) and an ultralow decay rate of less than
0.039 % in 1000 continuous cycling measurements. More impressively, the areal capacity can arrive as
high as 5.2 mAh cm-2 at a large sulfur loading of 7.8 mg cm-1 in lean electrolyte with a ratio of electrolyte
to sulfur (E/S) ratio of 6.8 ml g-1. Our work suggests that single crystalline few-layer borophene sheets
hold great potential for high-eciency Li-S batteries.
Results And Discussions
The LTLE synthesis process of few-layer borophene sheets is schematically illustrated in Fig.1a. By
optimizing the sonication power and solvent concentration, massive production of 2D borophene sheets
has been successfully achieved. N-methyl pyrrolidone (NMP) was found to be the most effective among
a series of solvents adopted in our experiment, as seen in Supplementary Figs.1 and 2. The color of the
product solution is dark-brown or dark-black in Fig.1b, varying with the sheet concentration. Moreover, the
mass of 2D sheets reaches as high as 10 mg and their yield is over 20 %, evidently increased compared
with previous reports ( 10 %48,49). The low-temperature approach is thus believed to improve the
exfoliation eciency of non-layered bulk materials, because of which signicantly enhances the
anisotropy discrepancy between the in-plane and out-of-plane covalence bonds50. Scanning electron
microscope (SEM) and atomic force microscope (AFM) images of the as-grown products are respectively
in Fig.1c, d, where ultrathin 2D borophene sheets are observed to have an edge length of 2 ~ 5 µm and
exhibit uniform and smooth appearance. The thickness of 2D sheet is observed to be only 1.32 ~ 2.32
nm, suggesting its ultra-thin nature.
X-ray diffraction (XRD) pattern of 2D borophene sheets (Supplementary Fig.3) shows the same
characteristic diffraction pattern as that of the theoretically calculated
β
12-borophene using density
functional theory (DFT), clearly different from that of the bulk
β
-rhombohedral boron powder (JCPDS No.
00-031-0207). As seen in Fig.1e, the X-ray photoelectron spectrum (XPS) of B 1s core level is consisted of
two characteristic components, attributed to the B-B species at 187.5 eV51 and the B-O species at 189.1
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eV52, respectively. And the molar ratio of the B-B to B-O species is estimated to be more than 94 %,
suggesting a majority of pure boron composition in 2D borophene sheets. The minor B-O species are
supposed to originate from the edge oxidation of borophene sheets during the short exposure to the air
after being taken out for XPS measurements (Supplementary Fig.4)52.
Raman spectroscopy was employed to better differentiate the 2D borophene sheets from the bulk boron
powders53,54. Four Raman peaks of 2D borophene sheets are clearly identied (Fig.1f) as the ngerprints
of
β
12 phase55, differing from those of bulk boron with
β
-rhombohedral phase. Accordingly, the strong
peak at ~ 268 cm− 1 is ascribed to the out-of-plane bending vibration mode of
β
12 phase55.
And the other peaks at ~ 423, ~901 and ~ 1017 cm− 1 are respectively indexed as the , and
modes, resulting from the in-plane stretching modes of
β
12 phase55.
Transmission electron microscopy (TEM) was performed to determine the surface conguration of 2D
borophene sheets. A typical TEM image in Fig.2a exhibits a similar planar morphology in line with the
aforementioned SEM and AFM results (Fig.1c, d). Close examination (Fig.2a inset) reveals an ultrathin
thickness of only 6 atomic layers with an adjacent planar distance of 5.1 Å. The high-resolution TEM
(HRTEM) image further veries high-quality single crystal nature of 2D borophene sheets. As shown in
Fig.2b, the 2D borophene sheets are found to have a hexagonal honeycomb lattice with a perfect planar
periodicity of
a
 = 
b
 2.76 Å and the intersection angle
θ
of about 120° in the unit cell. Based on the DFT
calculations, we thus propose a novel allotrope with symmetry (referred to as
β
12-B5) for few-
layer
β
12-borophene sheets, where there are 5 boron atoms in a unit cell (Fig.2c). In this model, both of
the lattice constants (
a
and
b
) of few-layer borophene are 2.83 Å and the angle
θ
between
a
and
b
vectors
is equal to 120°, which are in good agreement with the HRTEM results (Fig.2b). Based on the theoretical
model, the bright contrasts in the HRTEM image (Fig.2b) thus correspond to the six-member rings of the
boron honeycomb lattice (Fig.2c). Besides, the layer distance of adjacent (001) planes is theoretically
calculated to be about 5.0 Å for
β
12-B5 borophene, nearly identical to the experimental results (5.1 Å)
measured by TEM. Statistically, the thickness of most of the 2D sheets is less than 5 nm. (Supplementary
Fig.5a). Therefore, the atomic layer numbers of the as-synthesized borophene sheets should be less than
10, unveiling the ultrathin nature of few-layer borophene. In addition, the 2D
β
12-borophene sheets are
thermodynamically stable as evidenced by the absence of any negative frequency in the entire Brillouin
zone (Fig.2d) according to the density functional perturbation theory (DFPT). Similar calculations are
carried out on the (104) plane (Supplementary Fig.5), which are also in good agreement with our
experimental results. High-angle annular dark-eld scanning transmission electron microscope (HAADF-
STEM) and energy dispersive X-ray spectroscopy (EDX) mapping (Fig.2e-h) images also reveal a uniform
distribution of boron element across the 2D sheet with a pure boron content over 98 %, which is in good
consistent with the electron energy loss spectrum (EELS) (Supplementary Fig.5b).
Based on all characterizations mentioned above, we can thus conclude that single crystalline few-layer
borophene sheets with
β
12-B5 phase were successfully synthesized using LTLE. In comparison with other
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synthetic methods summarized in Supplementary Table1, our route is thus low-cost, facile and high-
ecient for scalable production of single crystalline
β
12-borophene sheets towards practical applications
such as Li-S battery.
To demonstrate the catalytic activity of few-layer
β
12-borophene sheets for Li-S battery, the potentiostatic
experiments were carried out to monitor the liquid-solid conversion in the nucleation and growth of Li2S
from polysuldes. The galvanostatic discharge was respectively performed on CNT/
β
12-borophene
(CNT/B) and bare CNT hosts at 2.05 V, in which 0.01 V overpotential was used to induce the generation
of Li2S. All the cells reached the highest potentiostatic current after about 1000 s, but the nucleation
abilities of Li2S were found to be completely different, causing a capacity of 193 mAh g− 1 and 72 mAh
g− 1 for CNT/B and CNT electrodes, respectively (Fig.3a). Besides, the dissolution ability of solid Li2S was
also remarkably promoted by the implantation of few-layer
β
12-borophene. After the fully conversion of
sulfurs into Li2S, the Li2S dissolution was kinetically evaluated by using a potentiostatic charge process.
Clearly, a larger oxidation current was detected on CNT/B (0.13 mA cm− 2) enabled cell in comparison
with bare CNT electrode (0.11 mA cm− 2), unveiling the excellent electrocatalysis behaviors of 2D
β
12-
borophene in enhancing the dissolution of Li2S (Fig.3b).
To gain insight into the enhancement effect of
β
12-borophene sheets on the liquid-liquid conversion
process (Li2Sy to Li2Sx, 8  x 2, 8 y 2), Li2S8 symmetric cells were employed for the cyclic
voltammetry (CV) measurements. The CNT/B-based cell yielded a higher redox current than the bare CNT-
based cell, suggesting enhanced reactivity of the polysulde on
β
12-borophene interface (Fig.3c). The
kinetic-regulating role of
β
12-borophene was subsequently demonstrated in actual Li-S batteries. The CV
curves of the as-assembled Li-S batteries exhibited two typical redox peaks, corresponding to the
formation of soluble polysuldes (2.2–2.4 V) and solid Li2S (2.0-2.1 V), respectively. And the two
overlapped anodic peaks (2.4–2.6 V) were attributed to the sequential oxidation of Li2S and
polysuldes29. In contrast with the Li-S battery using non-undecorated CNT electrode, the Li-S battery
using CNT/B electrode possessed higher current density (Fig.3d). As shown in Fig.3e, the Tafel plots of
the rst oxidation process of the cells using CNT and CNT/B were respectively 57 and 29 mV dec− 1,
where the smaller Tafel slope of the CNT/B-based cell suggests that the
β
12-borophene induces higher
surface reaction rates. In addition, the simulated interfacial impedance of the Li-S cells sharply decreased
from 41.2 to 24.9 when the electrodes changed from CNT to CNT/B (Fig.3f), reecting the
β
12-
borophene was more favorable for the interface electrochemical reactions8.
Considering the distinguished electrocatalytic reactivity and polysulde interactions of the CNT/B-based
Li-S battery in the sulfur redox reactions, their actual working performances were further evaluated by
regarding the bare CNT-based Li-S battery as a reference. In our experiments, the same amount of
polysulde (Li2S8) solution was added as active material (Supplementary Fig.6). As seen in Fig.4a, the
CV proles of CNT/B-based battery overlap each other and exhibit excellent reversibility in the redox
process, revealing the high-eciency utilization of sulfur. The galvanostatic charge/discharge proles are
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shown in Fig.4b. The high reversible specic capacities of 1329, 1236, 1159, 1057, 993, and 919 mAh g− 
1 were obtained at 0.3, 0.5, 1, 2, 3, and 5 C rates (1 C = 1675 mAh g− 1), respectively. Even if the current
density increased to 8 C, the CNT/B-based Li-S battery still remained an ultrahigh capacity of 721 mAh g− 
1. More signicantly, after returning current density back to 0.3 C, a reversible capacity of 1216 mAh g− 1
recovered immediately with a columbic eciency of nearly 100 % (Fig.4c). By contrast, the battery using
bare CNT electrode exhibited inferior rate performances, such as a lower initial capacity of 981 mAh g− 1
at 0.3 C and a rapider degradation into 394 mAh g− 1 with the increase of capacity to 8 C as well as
unsatisfactory capacity restoration after high-rate test (Fig.4c). As shown in Fig.4d, the CNT/B-based
battery possessed a much lower polarization voltage of 188 mV than the CNT-based battery (217 mV),
further revealing the outstanding catalytic property of
β
12-borophene sheets for polysulde conversion.
The CNT/B cathode also exhibited excellent cycling stability at current density of 0.5 C, as found in
Fig.4e. The capacity fading rate was only 0.003% per cycle and kept nearly unvaried after 300 cycles
when the initial capacity of the CNT/B-based Li-S battery was 1110 mAh g− 1. Moreover, the CNT/B-based
cell maintained a high coulombic eciency of ~ 100% in continuous 300 cycle measurements. On the
contrary, the bare CNT-based cell delivered a low capacity of 918 mAh g− 1 and sharply decreased to 394
mAh g− 1 after 300 cycles, resulting in a fast-fading rate of 0.2 % per cycle (Fig.4e and Supplementary
Fig.7). In addition, both of the high- and low-plateau capacities of CNT/B electrode were much better
than bare CNT electrode, demonstrating few-layer
β
12-borophene sheets can effectively suppress the
polysulde diffusion and improve the polysulde immobilization (Supplementary Fig.9)56. Moreover,
high areal sulfur loadings of 5.3 mg cm− 2 and 7.8 mg cm− 2 with low E/S ratios of 9.8 and 6.8 ml g− 1
were respectively performed on the Li-S batteries to test the high-energy density behaviors. It was noted
that the areal capacities of the CNT/B-based Li-S cells can reach up to 4.6 and 5.2 mAh cm− 2 when the
capacities respectively adopted 871 and 661 mAh g− 1 (Fig.4f), which were much higher than those of 4.0
mAh cm− 2 for commercial Li-ion batteries57. Impressively, the CNT/B-based battery could preserve an
enough high reversible capacity of 572 mAh g− 1 with an extremely-low capacity decay rate of 0.039 % per
cycle after 1000 long-term cycles, reecting excellent cycling stability (Fig.4g and Supplementary Fig.8).
Notably, the ultralow decay rate and ultrahigh rate performance of 2D
β
12-borophene sheets are superior
to most of other 2D material-based Li-S batteries (Supplementary Table2), such as phosphorene (785
mAh g− 1 at 3 C, decay rate of 0.053% for 1000 cycles)30, C3N4 (340 mAh g− 1 at 4 C, decay rate of 0.5%
for 200 cycles)58, and graphene (700 mAh g− 1 at 2 C, decay rate of 0.5% for 70 cycles)59.
Finally, we calculated the adsorption energy of soluble polysuldes on a monolayer
β
12-borophene using
DFT calculation to comprehend the improvement mechanism of
β
12-borophene sheets on Li-S batteries,
as observed in Supplementary Fig.10. Figure5a gives the optimized congurations of S8 and Li2Sn on
monolayer
β
12-borophene sheet. Based on the DFT calculations, S8 has the weakest adsorption energy on
borophene of only 1.23 eV among all congurations, and the adsorption energy gradually increases with
the progression of the polysuldes’ lithiation and eventually arrives at 3.8 eV for the fully-lithiated Li2S
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(Fig.5b). The adsorption energy of polysuldes on
β
12-borophene is far higher than that on CNT (below 1
eV), unveiling that
β
12-borophene can anchor polysulde and inhibit the shuttle of lithium polysulde
more effectively than CNT. Figure5c shows the typical partial density of states (PDOS) of Li2S4 on
monolayer
β
12-borophene, and more details can be seen in Supplementary Fig.11. The 2p orbital
electrons of Li2S4 and
β
12-borophene were found to overlap near the Fermi level, suggesting the
formation of a strong chemical bonding between
β
12-borophene and Li2S4 cluster. This is probably
originated from a strong charge transfer of 0.22 e from
β
12-borophene to Li2S4 cluster (Fig.5d) based on
the charge density difference and bader charge analysis. The strong chemical interaction between
β
12-
borophene and Li2S4 cluster can be also ascertained because the dark-yellow color of Li2S4 solution will
gradually attenuate with the increase of the mixing time with
β
12-borophene (Supplementary Fig.12).
Furthermore, the diffusion barrier of Li+ on
β
12-borophene was deduced to be only 0.10 eV (Fig.5e), much
lower than that (0.28 eV) on CNT (Supplementary Fig.13). The enhanced surface migration of Li+ on
borophene would further accelerate the nucleation and decomposition of Li2Sn and thus improves the
capacity and charge-discharge rate of Li-S battery60.
In summary, we have developed a novel, facile and high-yield LTLE strategy to produce single crystalline
few-layer
β
12-borophene sheets. As promising 2D electrode materials, the
β
12-borophene sheets were
rstly used as ecient polysulde-conversion electrocatalysts for Li-S batteries. Due to the usage of few-
layer
β
12-borophene sheets, the CNT/B
-
based Li-S batteries exhibited a high areal sulfur loading of 5.2
mgh cm− 2 at 7.8 mg cm− 2 under a low E/S ratio of 6.8 ml g− 1 at 0.3 C. Compared with the CNT-based Li-
S cell, the CNT/B-based Li-S cell exhibited a better rate performance of as high as 721 mAh g− 1 at 8 C
and a much lower decay rate of only 0.039 % in 1000 cycles. By DFT calculations,
β
12-borophene had a
lower surface diffusion barrier of Li ion and a stronger adsorption for Li2Sn clusters than CNT, which can
effectively inhibit the shuttle effect of polysuldes and accelerate their decomposition at the same time.
These should be responsible for the extraordinary catalytic activity of
β
12-borophene towards polysuldes
in the CNT/B-based Li-S cell. Therefore, our strategy will pave a new way for the design of high-energy
rechargeable batteries through the exploration of 2D boron-based nanomaterials.
Methods
Synthesis of few-layer β12-borophene sheets. The low-temperature liquid exfoliation (LTLE) method was
rstly developed to synthesize
β
12-borophene sheets at milligram scale by using boron powder (99.8 %,
Zhongnuo Incorp., China) as source materials. Firstly, 20 ~ 50 mg boron powers were added into 50 ml
N,N-Dimethylformamide (NMP, 99.9 %, Innochem. Incorp., China) to form uniform and well-dispersed
solution by several minutes’ stirring, as seen in Figure S1. Secondly, the boron-power solution was
transferred into the ethanol path and treated at -20~-25 in the tip-type ultrasonicator equipped with
cooling system (SXSONIC Incorp., China), where the ultrasonic power was kept at 800 W and the
treatment lasted for 4 ~ 8 h. Thirdly, the product solution was statically settled at room temperature for
48 ~ 72 h to enough precipitate the undissolved boron powder. Finally, the suspension was centrifuged at
Page 9/16
about 10000 ~ 11000 revolutions per minute (rpm) for 30 minutes to obtain solid products. After the
above synthesis process, the mass of the collected 2D sheets was ranging from 4 to 10 mg. Accordingly,
the yield of 2D few-layer borophene by LTLE way can reach as high as over 20 %, which is much higher
than those by many other methods in previous reports (Supplementary Table1) 48,49.
Material characterizations. The morphology of
β
12-borophene sheets was investigated by SEM (Zeiss
Supra 60) and AFM (Bruker Dimension Fastscan). XPS (Thermosher Nexsa), XRD (D-MAX 2200 VPC)
and Raman spectroscope (inVia Reex, 532-nm laser) were respectively used to analyze the chemical
compositions of the sample. UV-vis spectroscopy (UV-3600) was applied to determine the energy-band
structure and absorption coecient of
β
12-borophene sheets. TEM and HRTEM (FEI Titan 80–300) were
employed to ascertain the lattice structure of the product. The STEM and elemental mapping were
performed on a JEM ARM200F thermal-eld emission microscope with a probe Cs-corrector working at
200 kV. For the HAADF imaging, the convergence angle of ~ 23 mrad and collection angle range of 68 ~ 
174 mrad were adopted for the incoherent atomic number imaging. Both the elemental composition and
distribution were analyzed on the energy dispersive X-ray analyzer (EDS, EX-230 100m2 detector)
equipped with the microscope.
Preparation of Li 2 S 8 catholyte. The sources of sulfur and Li2S with a molar ratio of 7:1 were put into an
appropriate amount of 1 mol l− 1 lithium bis (triuoromethanesulfonyl) imide (LiTFSI). Secondly, the LiTFI
solution was added into the mixed solvent of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME)
(volume ratio (
v:v
): 1:1). Thirdly, 1
wt.
% LiNO3 was used as additive by vigorous magnetic stirring at 50
until the sulfur powder were fully dissolved. The concentration of Li2S8 was ranging from 0.15 to 2
mol l− 1.
Fabrication of CNT electrodes. 5 g commercial multiwalled CNT (length ~ 50 nm, Aladdin, China) powders
were dispersed into 50
ml
Triton X-100 aqueous solution (0.01
wt.
%, Secco Romeo, China) to form
uniform and homodisperse solution by ultrasonication for 2 h. Subsequently, the obtained CNT solution
was ltered through nylon lm under vacuum. After three-time washing by deionized water and drying for
2 h at 60 in vacuum oven, the free-standing CNT paper was peeled from the nylon lm. Finally, the
obtained CNT paper was cut into desired disks as the free-standing electrode.
Assembly of symmetric cells for kinetic evaluation of polysulde conversion. CNT/B (with a mass
loading of about 1 mg
β
12-borophene sheets) or bare CNT electrodes were used as both working and
counter electrodes. And 40 µl catholyte (0.5 mol l− 1 Li2S6 and 1.0 mol l− 1 solution of LiTFSI with 1
wt.
%
LiNO3 in DOL and DME,
v/v
 = 1:1) was added into each coin cell. The CV behaviors of the symmetric cell
were tested at a scan rate of 10 mV s− 1, in which the voltage window ranged from − 0.8 to 0.8 V.
Measurement on the nucleation and dissolution of Li 2 S. The CNT/B or CNT lm electrodes were used as
cathodes and Li foils were employed as the anodes. Also, 20
µl
Li2S8 solution (0.15 mol l− 1) was applied
as catholyte, and 20 µl electrolyte without Li2S8 was used as anolyte. For the nucleation and growth of
Page 10/16
Li2S, the assembled cells were rst discharged galvanostatically to 2.06 V at 0.112 mA, and then
discharged potentiostatically to 2.05 V until the current dropped to below 10− 5 A. The deposition
capacities of Li2S were calculated according to the Faraday’s law. For the Li2S dissolution, the assembled
cells were rstly galvanostatically discharged to 1.80 V at 0.10 mA, and subsequently galvanostatically
discharged to 1.80 V at 0.01 mA for fully transforming sulfur species into solid Li2S. Then the cells were
potentiostatically charged at 2.40 V to oxidize Li2S into soluble polysuldes. The potentiostatic charge
was accomplished when the charge current was below 10− 5 A.
Assembly and performance evaluation of Li–S cells. CR-2016 coin cells were assembled in an argon-
protected glove box, where the CNT/B or CNT lms were employed as the cathodes and 20 µl Li2S8
catholyte was dropped onto the CNT lm as the sulfur cathode. Also, Li foil was applied as the counter
electrode, and 1.0 M solution of LiTFSI with 1
wt.
% LiNO3 in DOL and DME (
v/v
 = 1:1) was used as the
electrolyte. In experiments, the common areal loading of sulfur was about 1 mg cm− 2, and the
electrolyte/sulfur ratio was xed at 15 µl mg− 1. The electrochemical performances of Li-S batteries were
measured by a LANDCT2001A analyzer, where the voltage interval ranged from 1.7 to 2.8 V. And the
cyclic voltammograms (CV) curves were collected at 0.1 mV s− 1on a CHI-760E electrochemical
workstation (Chenhua Instrument, Shanghai), in which EIS analysis was in the range of 10 kHz-0.01 Hz.
Theoretical model of few-layer β12-borophene sheets. All the calculations except superconducting
properties were carried out using Vienna
ab initio
simulation package (VASP 5.4)61,62 with projector
augmented wave (PAW) pseudopotential method63,64 and Perdew-Burke-Ernzerhof (PBE) functional65.
Both lattice parameters and atomic positions were optimized by conjugate gradient method, and the
convergence criteria for energy and force were eV and eVÅ-1, respectively. The
kinetic energy cutoff for plane waves was set at 450 eV. The Brillouin zones were sampled with
Å-1 spacing in reciprocal space by the Monkhorst-Pack scheme66. The high symmetry K-
points for band structure and phonon dispersion curves were generated by AFLOW package67. And
Grimmes DFT-D3 van der Waals corrections with the Becke-Jonson damping68,69 was employed. The
phonon spectrum was calculated by DFPT method implemented in Phonopy program69. Also, the crystal
structures were visualized by VESTA package70.
Computational methods of the adsorption energy of few-layer β12-borophene. First-principle calculations
were implemented using VASP61 software package. The PBE65 functional of generalized gradient
approximation (GGA) was used for the exchange-correlation. The basis set utilized PAW pseudopotential
method63,64, and the energy cutoff was set at 400 eV. The self-consistent eld (SCF) tolerance was
eV and the force convergence criterion for atomic relaxation was 0.02 eV Å−1. A Monkhorst-
Pack k-point mesh with different sizes was chosen to meet various requirements, where is
for the geometrical relaxation, is for the calculation of electronic structure and
is for the calculation of adsorption. The vdW forces between Li2Sn and
β
12-borophene sheet or CNT were
accurately obtained by the DFT-D3 method68. The supercell of
β
12-borophene and
Page 11/16
CNT was used for the adsorption energy and CI-NEB calculations, respectively. The adsorption energy (
) was derived using the following equation:
Data availability
The authors declare that all the data supporting the ndings of this study are available within the article
and its Supplementary Information or from the corresponding authors upon reasonable request.
Declarations
Acknowledgements
The authors are very thankful for the support of the National Science Foundation of China (Grant Nos.
51872337, 51872283, 22075279, 21872145), National Project for the Development of Key Scientic
Apparatus of China (2013YQ12034506), National Key Research and Development Program of China
(Grant no. 2019YFA0210203, 2016YFB0100100, 2016YFA0200200), the Fundamental Research Funds
for the Central Universities of China, the Science and Technology Department of Guangdong Province
and the Education Department of Guangdong Province, the Liao Ning Revitalization Talents Program
(Grant XLYC1807153), the Natural Science Foundation of Liaoning Province, Joint Research Fund
Liaoning-Shenyang National Laboratory for Materials Science (Grant 20180510038), Dalian Science and
Technology Bureau (2019RT09), Dalian National Laboratory For Clean Energy (DNL), CAS, DNL
Cooperation Fund, CAS (DNL180310, DNL180308, DNL201912, and DNL201915), DICP (DICP
ZZBS201708, DICP ZZBS201802, DICP I2020032), DICP&QIBEBT (Grant DICP&QIBEBT UN201702),
GFKJCXTQ Foundation (Grant 18-163-14-ZT-002-001-02).
Author contributions
Page 12/16
B. Y., Z. -S. W. and F. L. proposed and supervised the projects. H.J.L. synthesized the borophene sheets,
and characterized their surface morphology and chemical compositions. H. D. S. fabricated the CNT- and
CNT/B-based Li-S batteries, and carried out the electrochemical measurements. Z. W. carried out the TEM
analysis of the borophene sheets and calculated the adsorption energies of Li2Sn clusters on monolayer
borophene or CNT by DFT model. Y. W. M. proposed the DFPT model of the surface conguration of the
β
12-borophene. H. J. L., H. D. S, Z. W., Y. W. M., S. D. L., B. Y., Z. -S. W. and F. L. wrote the paper. All the
authors involved in the analysis and discussion of the experimental results. And all authors approve to
submit the nal version of the manuscript.
Additional information
Supplementary Information accompanies this paper at http://www.nature.com/naturecommunications
Competing nancial interests: The authors declare no competing nancial interests
Reprints and permissioninformation is available online at http://npg.nature.com/ reprints and
permissions/
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps
and institutional aliations
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Freestanding, robust electrodes with high capacity and long lifetime are of critical importance to the development of advanced lithium–sulfur (Li–S) batteries for next‐generation electronics, whose potential applications are greatly limited by the lithium polysulfide (LiPS) shuttle effect. Solutions to this issue have mostly focused on the design of cathode hosts with a polar, sulfurphilic, conductive network, or the introduction of an extra layer to suppress LiPS shuttling, which either results in complex fabrication procedures or compromises the mechanical flexibility of the device. A robust Ti3C2Tx/S conductive paper combining the excellent conductivity, mechanical strength, and unique chemisorption of LiPSs from MXene nanosheets is reported. Importantly, repeated cycling initiates the in situ formation of a thick sulfate complex layer on the MXene surface, which acts as a protective membrane, effectively suppressing the shuttling of LiPSs and improving the utilization of sulfur. Consequently, the Ti3C2Tx/S paper exhibits a high capacity and an ultralow capacity decay rate of 0.014% after 1500 cycles, the lowest value reported for Li–S batteries to date. A robust prototype pouch cell and full cell of Ti3C2Tx/S paper // lithium foil and prelithiated germanium are also demonstrated. The preliminary results show that Ti3C2Tx/S paper holds great promise for future flexible and wearable electronics.
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The lithium sulfur (Li–S) battery is a preferential option for next‐generation energy storage technologies, but the lithium polysulfide shuttling, sluggish redox kinetics, and uncontrollable lithium dendrite growth hamper its commercial viability. Herein, well‐dispersed single atom Zn‐decorated hollow carbon spheres (Zn1‐HNC) are developed as dual‐functional nanoreactors for polysulfides‐suppressed sulfur cathodes (Zn1‐HNC‐S) and dendrite‐free lithium anodes (Zn1‐HNC‐Li) simultaneously for high‐capacity, high‐rate, and long‐cycling Li–S batteries with fast redox kinetics. Benefiting from its excellent electronic conductivity, high surface area (370 m² g⁻¹), highly‐effective active sites and protective carbon shell, the resultant nanoreactor possesses strong physical confinement, chemical anchoring, and exceptional electrocatalysis for polysulfides. Meanwhile, the nanoreactor with excellent lithiophilic ability realizes uniform and dendrite‐free lithium deposition. Integrating all these advantages, the assembled full battery (Zn1‐HNC‐S||Zn1‐HNC‐Li) delivers remarkable electrochemical properties including long cycle stability with an ultralow capacity fading rate of 0.015% per cycle over 700 cycles and superb rate performance of 989 mAh g⁻¹ at 10 C. Moreover, a high areal capacity of 8.7 mAh cm⁻² with high S loading of 7.8 mg cm⁻² at low E/S ratio (6.4 µL mg⁻¹) is achieved. This work provides significant insight of structure and surface catalytic chemistry regulation for promoting the actual application of Li–S batteries.
Article
Guiding the lithium‐ion (Li‐ion) transport for homogeneous, dispersive distribution is crucial for dendrite‐free Li anodes with high current density and long‐term cyclability, but remains challenging for the unavailable well‐designed nanostructures. Herein, we propose a two‐dimensional (2D) heterostructure composed of defective graphene oxide (GO) clipped on mesoporous polypyrrole (mPPy) as a dual‐functional Li‐ion redistributor to regulate the stepwise Li‐ion distribution and Li deposition for extremely stable, dendrite‐free Li anode. Owing to the synergy from Li‐ion transport nanochannels of mPPy and Li‐ion nanosieves of defective GO, 2D mPPy‐GO heterostructure achieves ultralong cycling stability (1000 cycles), even test at 0 and 50 o C, ultralow overpotential of 70 mV at high current density of 10.0 mA cm ‐2 , outperforming most of reported Li anodes. Furthermore, mPPy‐GO‐Li/LiCoO 2 full batteries demonstrate remarkably enhanced performance with capacity retention of > 90% after 450 cycles. Therefore, this work opens many opportunities for creating 2D heterostructures for high‐energy‐density Li metal batteries.
Article
Amruth Bhargav is a PhD student in the Materials Science and Engineering graduate program at the University of Texas at Austin. He obtained his B.E. in Mechanical Engineering from Visvesvaraya Technological University, India, in 2013 and a Master of Science in Mechanical Engineering from Indiana University-Purdue University Indianapolis in 2016. His research mainly focuses on lithium-sulfur and lithium-organosulfur batteries. Dr. Jiarui He is a postdoctoral fellow in the Texas Materials Institute at the University of Texas at Austin. He obtained his B.E. (2012) and his PhD (2018) in electronic information materials and devices from the University of Electronic Science and Technology of China. His research interests are in the area of electrochemical conversion and storage materials. He has authored more than 76 publications, with 3,000 citations and an h-index of 33 (Google Scholar). Abhay Gupta is a PhD student in the Materials Science and Engineering graduate program at the University of Texas at Austin. He received his bachelor’s degree from the Hildebrand Department of Petroleum and Geosystems Engineering at the University of Texas at Austin in 2016. His research mainly focuses on lithium-sulfur batteries with an emphasis on low-temperature performance. Professor Arumugam Manthiram is the Cockrell Family Regents Chair in engineering and the Director of Texas Materials Institute and the Materials Science and Engineering program at the University of Texas at Austin. His research interests are in the area of materials for rechargeable batteries and fuel cells, including novel synthesis approaches, advanced characterization, and prototype device fabrication. He has authored more than 800 publications, with 60,000 citations and an h-index of 123 (Google Scholar). See https://www.sites.utexas.edu/manthiram for further details.
Article
Li-S batteries have several advantages in terms of ultrahigh energy density and resource abundance. However, the insulating nature of S and Li2S, solubility and shuttle effect of lithium polysulfides (LiPSs), and slow interconversion between LiPSs and S/Li2S/Li2S2 are significant impediments to the commercialization of Li-S batteries. Exploration of advanced S host skeleton simultaneously with high conductivity, adsorbability and catalytic activity is highly desired. Herein, a heterojunction material with holey nanobelt morphology and low surface area (95 m2/g) is proposed as compact cathode host to enable a conformal deposition of S/Li2S with homogenous spatial distribution. The rich heterointerfaces between MoO2 and Mo3N2 nanodomains serve as job-synergistic trapping-conversion sites for polysulfides by combining the merits of conductive Mo3N2 and adsorptive MoO2. This non-carbon heterojunction substrate enables a high S loading of 75 wt% even under low surface area. The initial capacity of MoO2-Mo3N2@S reaches 1003 mAh/g with a small decay rate of 0.024% per cycle during 1000 cycles at 0.5 C. The long-term cyclability is preserved even under a high loading of 3.2 mg/cm2 with a reversible capacity of 451 mAh/g after 1000 cycles. The Li-ion diffusion coefficient for MoO2-Mo3N2@S is extremely high (up to 2.7×10-7 cm2/s) benefiting from LiPSs conversion acceleration at heterojunctions. The affinity between LiPSs and heterojunction allows a dendrite-free Li plating at anode even after long-term cycling. Well-defined heterointerface design with job-sharing or job-synergic function appears to be a promising solution to high-performance Li-S batteries without the requirement of loose or high-surface-area carbon network structures.
Article
Lithium–sulfur (Li–S) batteries have great potential as an electrochemical energy storage system because of the high theoretical energy density and acceptable cost of financial and environment. However, the shuttle effect leads to severe capacity fading and low coulombic efficiency. Here, graphitic carbon nitride (g–C3N4) is designed and prepared via a feasible and simple method from trithiocyanuric acid (TTCA) to anchor the polysulfides and suppress the shuttle effect. The obtained g–C3N4 exhibits strong chemical interaction with polysulfides due to its high N–doping of 56.87 at%, which is beneficial to improve the cycling stability of Li–S batteries. Moreover, the novel porous framework and high specific surface area of g–C3N4 also provide fast ion transport and broad reaction interface of sulfur cathode, facilitating high capacity output and superior rate performance of Li–S batteries. As a result, Li–S batteries assembled with g–C3N4 can achieve high discharge capacity of 1200 mAh/g at 0.2 C and over 800 mAh/g is remained after 100 cycles with a coulombic efficiency more than 99.5%. When the C–rate rises to 5 C, the reversible capacity of Li–S batteries can still maintain at 607 mAh/g.